if_vtnet - Fix potential vtnet_set_hwaddr call before virtqueues are set up.

if_vtnet - Fix potential vtnet_set_hwaddr call before virtqueues are set up.

* When VIRTIO_NET_F_MAC isn't negotiated (i.e. the host doesn't give us
  a macaddress), we generate a random mac address. The vtnet_set_hwaddr()
  call to set this mac address, would have tried to use the control
  virtqueue too early, where it isn't yet allocated.

* To fix this case, do the vtnet_set_hwaddr() call for this a bit later,
  after the virtqueues have been set up. (We are already disabling
  promiscuous mode there, so we know the controlq is working there)